1. Field of the Invention
This disclosure relates in general to magnetic storage systems, and more particularly to a method and apparatus for providing a write head with an improved pole tip to improve overwrite and/or adjacent track interference (OW/ATI).
2. Description of Related Art
There has been huge progress in the field of magnetic storage system technology in almost 50 years. Moreover, the rate of this progress is increasing year after year. Such success has made storage systems an important component of modern computers.
Some of the most important customer attributes of any storage system are the cost per megabyte, data rate, and access time. In order to obtain the relatively low cost of today's storage system compared to solid state memory, the customer must accept the less desirable features of this technology, which include a relatively slow response, high power consumption, noise, and the poorer reliability attributes associated with any mechanical system. On the other hand, magnetic storage systems have always been nonvolatile; i.e., no power is required to preserve the data, an attribute which in semiconductor devices often requires compromises in processing complexity, power-supply requirements, writing data rate, or cost.
Improvements in areal density have been the chief driving force behind the historic improvement in storage cost. In fact, the areal density of magnetic storage systems continues to increase. While nature allows us to scale down the size of each bit of information, it does not allow scaling to happen forever.
Today, as the magnetic particles that make up recorded data on a storage system become ever smaller, technical difficulties in writing and reading such small bits occur. Further, as areal density increases, the requirements put on head designs will change.
In a magnetic head, a read element and a write element are formed having an air bearing surface ABS, in a plane, which can be aligned to face the surface of the magnetic disk. The read element includes a first shield, a second shield, and a read sensor that is located within a dielectric medium between the first shield and the second shield. The most common type of read sensor 48 used in the read/write head 28 is the magnetoresistive (AMR or GMR) sensor, which is used to detect magnetic field signal changes in a magnetic medium by means of changes in the resistance of the read sensor imparted from the changing magnitude and direction of the magnetic field being sensed.
The write element is typically an inductive write element that includes the second shield that functions as a first pole for the write element and a second pole disposed above the first pole. The first pole and the second pole contact one another at a backgap portion, with these three elements collectively forming the yoke. The combination of a first pole tip portion and a second pole tip portion near the ABS are sometimes referred to as the ABS end 56 of the write element. Some write elements have included a pedestal that can be used to help define track width and throat height. A write gap is formed between the first and second poles in the area opposite the back gap portion. The write gap is typically filled with a non-magnetic, electrically insulating material that forms a write gap material layer. A conductive coil passes through the yoke. The write head operates by passing a writing current through the conductive coil. Because of the magnetic properties of the yoke, a magnetic flux is induced in the first and second poles by write currents passed through the coil. The write gap allows the magnetic flux to fringe out from the yoke thus forming a fringing gap field and to cross the magnetic recording medium that is placed near the ABS.
Areal density, also sometimes called bit density, refers to the amount of data that can be stored in a given amount of hard disk platter “real estate”. Since disk platters surfaces are of course two-dimensional, areal density is a measure of the number of bits that can be stored in a unit of area. It is usually expressed in bits per square inch (BPSI). Average minimum track spacing (or “ATS”) is a measure of how close adjacent data tracks can be positioned to one another for a particular data head without degrading the position error margin. If the head is too wide due to manufacturing tolerances, the position error margin may be insufficient as a result of the head having an ATS value that is greater than the product track “pitch”. Track pitch is the actual distance between adjacent track centers. The “average” minimum track spacing is the average of the minimum calculated track spacing intercepts to the right and to the left of the ideal track position. There is a desire to constrain the average minimum track spacing for each data head to be less than the product track pitch.
Overwrite capability is the capability of the data head to write new data over previously written data on the disc surface. There is a desire to maximize the overwrite capability to minimize the previously written data's effect on the newly written data. Overwrite capability is measured by comparing the frequency content of an original data signal with the frequency content of the data signal after an overwrite condition involving a different write signal frequency.
Manufacturing tolerances incurred during the fabrication process of the data head may have adverse effects on the error rate performance. These manufacturing tolerances may result in pole tip saturation problems and marginally wide head situations, among other performance problems. Pole tip saturation occurs in the data head during write operations when additional magnetic field applied to the head results in no further increase in the data head's magnetization at the gap edge of the trailing pole tip. As a result, the media is recorded with a more gradually changing magnetic field which produces a more gradual transition in the media that is later transduced by the head in a read mode to form a broader pulse than usual. Broad pulses are more likely to cause errors in the read operation. Data heads having relatively short throat lengths are more likely to experience pole tip saturation than data heads having relatively long throat lengths.
Manufacturing tolerances also affect the effective width of the data head. If the data head is too wide, the magnetic field generated during write operations may adversely affect the data written on adjacent data tracks. During read operations, the data read from the disc surface by a wide head may be corrupted by adjacent track interference. If the data head is too narrow, the read signal amplitude may be too small. This adversely affects position error margin. In other words, the data head cannot get very far off track without creating an unacceptable level of bit errors in the data read from the data track.
Current recording heads are approaching a Track Per Inch (TPI) Density larger than 100K. The presents a technological challenge in balancing the overwrite and the adjacent track interference. Another challenge is the aspect ratio of the flare length/P2B is getting larger, which degrades the stress anisotropic and lowers the permeability of the pole tip. However, simply reducing flare length runs the risk of over saturating the pole tip.
It can be seen then that there is a need for a method and apparatus for providing a write head with an improved pole tip to improve overwrite and/or adjacent track interference.
FIG. 1 illustrates an exemplary storage system 100 according to the present invention. A transducer 110 is under control of an actuator 120, whereby the actuator 120 controls the position of the transducer 110. The transducer 110 writes and reads data on magnetic media 130. The read/write signals are passed to a data channel 140. A signal processor 150 controls the actuator 120 and processes the signals of the data channel 140 for data exchange with external Input/Output (I/O) 170. I/O 170 may provide, for example, data and control conduits for a desktop computing application, which utilizes storage system 100. In addition, a media translator 160 is controlled by the signal processor 150 to cause the magnetic media 130 to move relative to the transducer 110. The present invention is not meant to be limited to a particular type of storage system 100 or to the type of media 130 used in the storage system 100.
FIG. 2 illustrates one particular embodiment of a multiple magnetic disk storage system 200 according to the present invention. In FIG. 2, a hard disk drive storage system 200 is shown. The system 200 includes a spindle 210 that supports and rotates multiple magnetic disks 220. The spindle 210 is rotated by motor 280 that is controlled by motor controller 230. A combined read and write magnetic head 270 is mounted on slider 260 that is supported by suspension 250 and actuator arm 240. Processing circuitry exchanges signals that represent information with read/write magnetic head 270, provides motor drive signals for rotating the magnetic disks 220, and provides control signals for moving the slider 260 to various tracks. Although a multiple magnetic disk storage system is illustrated, a single magnetic disk storage system is equally viable in accordance with the present invention.
The suspension 250 and actuator arm 240 position the slider 260 so that read/write magnetic head 270 is in a transducing relationship with a surface of magnetic disk 220. When the magnetic disk 220 is rotated by motor 280, the slider 240 is supported on a thin cushion of air (air bearing) between the surface of disk 220 and the ABS 290. Read/write magnetic head 270 may then be employed for writing information to multiple circular tracks on the surface of magnetic disk 220, as well as for reading information therefrom.
FIG. 3 illustrates a sensor assembly 300. In FIG. 3, a slider 320 is mounted on a suspension 322. First and second solder connections 302 and 308 connect leads from the sensor 318 to leads 310 and 314, respectively, on suspension 322 and third and fourth solder connections 304 and 306 connect to the write coil (not shown) to leads 312 and 316, respectively, on suspension 322.
FIG. 4 is an ABS view of slider 400 and magnetic head 410. The slider has a center rail 420 that supports the magnetic head 410, and side rails 430 and 460. The support rails 420, 430 and 460 extend from a cross rail 440. With respect to rotation of a magnetic disk, the cross rail 440 is at a leading edge 450 of slider 400 and the magnetic head 410 is at a trailing edge 470 of slider 400.
The above description of a typical magnetic recording disk drive system, shown in the accompanying FIGS. 1-4, is for presentation purposes only. Storage systems may contain a large number of recording media and actuators, and each actuator may support a number of sliders. In addition, instead of an air-bearing slider, the head carrier may be one that maintains the head in contact or near contact with the disk, such as in liquid bearing and other contact and near-contact recording disk drives.
FIG. 5 is a side cross-sectional elevation view of a magnetic head 540. Note that FIG. 5 does not show the details of the pole tip region. The magnetic head 540 includes a write head portion 570 and a read head portion 572. The read head portion 572 includes a sensor 574. FIG. 6 is an ABS view of the magnetic head of FIG. 5. The sensor 574 is sandwiched between first and second gap layers 576 and 578, and the gap layers are sandwiched between first and second shield layers 580 and 582.
In a piggyback head as shown in FIG. 5, the second shield layer (S2) 582 and the first pole piece (P1) 592 are separate layers. The first and second shield layers 580 and 582 protect the MR sensor element 574 from adjacent magnetic fields. More conventionally, the second shield 582 also functions as the first pole (P1) 592 of the write element, giving rise to the term “merged MR head.” However, the present invention is not meant to be limited to a particular type of MR head.
In response to external magnetic fields, the resistance of the sensor 574 changes. A sense current Is conducted through the sensor causes these resistance changes to be manifested as voltage changes. These voltage changes are then processed as readback signals by the signal processing system 350 shown in FIG. 3.
The write head portion of the magnetic head includes a coil layer 584 sandwiched between first and second insulation layers 586 and 588. A third insulation layer 590 may be employed for planarizing the head to eliminate ripples in the second insulation layer caused by the coil layer 584. An insulation layer 542 is formed over the second pole piece layer 594. The first, second and third insulation layers are referred to in the art as an “insulation stack.” The coil layer 584 and the first, second and third insulation layers 586, 588 and 590 are sandwiched between first and second pole piece layers 592 and 594. The first and second pole piece layers 592 and 594 are magnetically coupled at a back gap 596 and have first and second pole tips 598 and 501 which are separated by a write gap layer 502 at the ABS 548. The first pole piece layer 592 is separated from the second shield layer 582 by an insulation layer 503.
FIG. 7 illustrates a view of the connect leads 520, 522 coupled to the coil 584 for the write pole piece 594. As shown in FIGS. 4-7, first and second solder connections 404 and 406 connect leads from the sensor 574 to leads 412 and 414 on the suspension 444, and third and fourth solder connections 416 and 418 connect leads 520 and 522 from the coil 584 (see FIG. 7) to leads 424 and 426 on the suspension.